2 Study Designs

7/29/2019 2 Study Designs

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Study designs


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Recall from previous session

Selection bias (sampling bias) (includingBerksons bias )

Non respondent bias / volunteer effect

Measurement bias (including Hawthorne effect) Experimenter expectancy bias

Lead time bias

Recall bias

Late look bias

Confounding bias


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Type of bias Definition Important

Associations

Solutions

Selection Sample not

representative

Berksons bias, Non

respondent bias

Random

independent

sample

Measurement Gathering the

information,

distorts it

Hawthorne effect Control group/

Placebo group

Experimenter

expectancy

Researchers belief

affects the outcome

Pygmalion effect

(on students)

Double blind

studies

Lead time Early detection in

screening confused

with increasedsurvival


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Question pattern 2 : What type of study design ?


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Types of study designs

Key distinction : Observational studies vsExperimental studies (clinical trials)

Observation InterventionObservational study : When the researcher simply

observes and records any characteristic. Eg.. Number

of teenagers who drop out of school or Number of

people dying of a particular disease etc.

Experimental study : When there is some sort of

intervention to the study group. Eg.. A new drug,

placebo, education, new surgery, new diagnostic test

etc..


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Exam guidance

These 2 broad types Observational andexperimental will further be divided into types orsubtypes.

However, when a question asks you to identifythe study design used, make sure that youidentify whether it is an observational orexperimental study first following this you will

have the option to eliminate any of the optionswhich belong to the other category Thus, afterthat even if you guess the name of a subtype, itwill be an intelligent guess with increased

peobability


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Types of Observational Studies

Case report : A brief objective report of a clinical

characteristic or outcome from a single clinical subjector event; n = 1. there is no control group.

- E.g., A 23 year old male with syphilitic lesions on the

skin.- Some of the most prestigious scientific journals like

The lancetpublishes a case report in every issue.

- It is the beginning of a possibility of something thatmay be occurring e.g. The first reported case of HIV orthe first reported case of hepatitis B. It all starts witha case report.


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Case series report: Objective report or outcome froma group of clinical subjects. N>1

Eg. Patients at a local hospital with syphilitic lesions.

No control group

- Looking for the common elements of a diseaseamongst people who have it.

- Eg.. As some of your books may say; the patientssuffering from angina experience chest pain referredto the shoulder, Nausea, dizziness etc.. These arethe common clinical elements seen for a disease in

various patients


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Next question to be answered How wide spread

is this disease?- Cross sectional study : A particular point in time

- The presence or absence of disease or othervariables are determined in each member of the

study population or a representative sample at aparticular time. The co-occurrence of a diseaseand a variable can be examined.

- Disease prevalence is recorded (NOT INCIDENCE)

- The temporal sequence of cause and effectcannot be determine

- There is no time frame.


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To increase our knowledge further we need to

know what causes, potentiates or facilitates the

disease process Case-control studies : Retrospective study of

disease vs non-disease.

Identifies a group of people with the disease andcompares them with a suitable comparison group

without the disease. Done retrospectively.

Incidence or prevalence CANNOT be assessed Can help determine causal relationships

Very useful for studying conditions with very low

incidence or prevalence


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Following this I would like to actually observe howthe disease occurs, the process, the factors and theeffect.

Cohort study : Prospective study of groupsfollowed over time.

Population group of those who have been exposed

to risk factor is identified and followed over timeand compared with a group not exposed to therisk factor.

- Prospective subjects tracked forward in time- Can determine incidence and causal relationship

- Must follow population long enough for incidenceto appear


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Therefore in a cohort study INCIDENCE DATA

CAN BE GENERATED

Not the Prevalence.

Therefore if you are given incidence data in a

question, it has to be a COHORT study

It is the best of all the observational study for

understanding a disease process

DOWN SIDE Time consuming and expensive


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Crosssectional

Case control Cohort


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Most feasible study for clinicians who wish to

do research ?

Case control study


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Analysis of the studies

For Cohort study :

Relative risk (RR) :-

Divide : Ie / Iu (I = Incidence, e = Exposed, u =

unexposed)

How much more likely

Absolute risk reduction :

Subtract : Ie - IuHow many more cases

Documents

2 Study Designs